The molecular nature of many genetic mutations resulting in deficiencies or electrophoretic variants of human LDH-A or LDH-B isozyme has been illustrated. The LDH-A deficiency is a latent heredity disorder of exertional myopathy. Female patients frequently complain of uterine stiffness during pregnancy, and delivery by Caesarean section is necessary. This information is of value to improve health care, particularly of women, and it will also allow more accurate evaluation of genetic mutation events caused by environmental mutagens.